Three-Dimensional Array of Re-Programmable Non-Volatile Memory Elements Having Vertical Bit Lines and a Single-Sided Word Line Architecture

ABSTRACT

A three-dimensional array especially adapted for memory elements that reversibly change a level of electrical conductance in response to a voltage difference being applied across them. Memory elements are formed across a plurality of planes positioned different distances above a semiconductor substrate. Bit lines to which the memory elements of all planes are connected are oriented vertically from the substrate and through the plurality of planes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/933,672, filed on Nov. 5, 2015, which is a continuation of U.S.application Ser. No. 14/153,794, filed on Jan. 13, 2014, now U.S. Pat.No. 9,190,134, which is a continuation of U.S. application Ser. No.13/735,983, filed on Jan. 7, 2013, now U.S. Pat. No. 8,780,605, which isa continuation of U.S. application Ser. No. 12/748,260, filed on Mar.26, 2010, now U.S. Pat. No. 8,351,236, which is a continuation-in-partof U.S. application Ser. No. 12/420,334 filed on Apr. 8, 2009, now U.S.Pat. No. 7,983,065.

BACKGROUND

The subject matter of this application is the structure, use and makingof re-programmable non-volatile memory cell arrays, and, morespecifically, to three-dimensional arrays of memory storage elementsformed on semiconductor substrates.

Uses of re-programmable non-volatile mass data storage systems utilizingflash memory are widespread for storing data of computer files, camerapictures, and data generated by and/or used by other types of hosts. Apopular form of flash memory is a card that is removably connected tothe host through a connector. There are many different flash memorycards that are commercially available, examples being those sold undertrademarks CompactFlash (CF), the MultiMediaCard (MMC), Secure Digital(SD), miniSD, microSD, Memory Stick, Memory Stick Micro, xD-PictureCard, SmartMedia and TransFlash. These cards have unique mechanicalplugs and/or electrical interfaces according to their specifications,and plug into mating receptacles provided as part of or connected withthe host.

Another form of flash memory systems in widespread use is the flashdrive, which is a hand held memory system in a small elongated packagethat has a Universal Serial Bus (USB) plug for connecting with a host byplugging it into the host's USB receptacle. SanDisk Corporation,assignee hereof, sells flash drives under its Cruzer, Ultra and ExtremeContour trademarks. In yet another form of flash memory systems, a largeamount of memory is permanently installed within host systems, such aswithin a notebook computer in place of the usual disk drive mass datastorage system. Each of these three forms of mass data storage systemsgenerally includes the same type of flash memory arrays. They each alsousually contain its own memory controller and drivers but there are alsosome memory only systems that are instead controlled at least in part bysoftware executed by the host to which the memory is connected. Theflash memory is typically formed on one or more integrated circuit chipsand the controller on another circuit chip. But in some memory systemsthat include the controller, especially those embedded within a host,the memory, controller and drivers are often formed on a singleintegrated circuit chip.

There are two primary techniques by which data are communicated betweenthe host and flash memory systems. In one of them, addresses of datafiles generated or received by the system are mapped into distinctranges of a continuous logical address space established for the system.The extent of the address space is typically sufficient to cover thefull range of addresses that the system is capable of handling. As oneexample, magnetic disk storage drives communicate with computers orother host systems through such a logical address space. The host systemkeeps track of the logical addresses assigned to its files by a fileallocation table (FAT) and the memory system maintains a map of thoselogical addresses into physical memory addresses where the data arestored. Most memory cards and flash drives that are commerciallyavailable utilize this type of interface since it emulates that ofmagnetic disk drives with which hosts have commonly interfaced.

In the second of the two techniques, data files generated by anelectronic system are uniquely identified and their data logicallyaddressed by offsets within the file. Theses file identifiers are thendirectly mapped within the memory system into physical memory locations.Both types of host/memory system interfaces are described and contrastedelsewhere, such as in patent application publication no. US 2006/0184720A1.

Flash memory systems typically utilize integrated circuits with arraysof memory cells that individually store an electrical charge thatcontrols the threshold level of the memory cells according to the databeing stored in them. Electrically conductive floating gates are mostcommonly provided as part of the memory cells to store the charge butdielectric charge trapping material is alternatively used. A NANDarchitecture is generally preferred for the memory cell arrays used forlarge capacity mass storage systems. Other architectures, such as NOR,are typically used instead for small capacity memories. Examples of NANDflash arrays and their operation as part of flash memory systems may behad by reference to U.S. Pat. Nos. 5,570,315, 5,774,397, 6,046,935,6,373,746, 6,456,528, 6,522,580, 6,643,188, 6,771,536, 6,781,877 and7,342,279.

The amount of integrated circuit area necessary for each bit of datastored in the memory cell array has been reduced significantly over theyears, and the goal remains to reduce this further. The cost and size ofthe flash memory systems are therefore being reduced as a result. Theuse of the NAND array architecture contributes to this but otherapproaches have also been employed to reducing the size of memory cellarrays. One of these other approaches is to form, on a semiconductorsubstrate, multiple two-dimensional memory cell arrays, one on top ofanother in different planes, instead of the more typical single array.Examples of integrated circuits having multiple stacked NAND flashmemory cell array planes are given in U.S. Pat. Nos. 7,023,739 and7,177,191.

Another type of re-programmable non-volatile memory cell uses variableresistance memory elements that may be set to either conductive ornon-conductive states (or, alternately, low or high resistance states,respectively), and some additionally to partially conductive states andremain in that state until subsequently re-set to the initial condition.The variable resistance elements are individually connected between twoorthogonally extending conductors (typically bit and word lines) wherethey cross each other in a two-dimensional array. The state of such anelement is typically changed by proper voltages being placed on theintersecting conductors. Since these voltages are necessarily alsoapplied to a large number of other unselected resistive elements becausethey are connected along the same conductors as the states of selectedelements being programmed or read, diodes are commonly connected inseries with the variable resistive elements in order to reduce leakagecurrents that can flow through them. The desire to perform data readingand programming operations with a large number of memory cells inparallel results in reading or programming voltages being applied to avery large number of other memory cells. An example of an array ofvariable resistive memory elements and associated diodes is given inpatent application publication no. US 2009/0001344 A1.

SUMMARY

The present application is directed to a three-dimensional array ofmemory elements wherein bit lines of the array are oriented vertically.That is, instead of merely stacking a plurality of existingtwo-dimensional arrays on a common semiconductor substrate, where eachtwo-dimensional array has its own bit lines, multiple two-dimensionalarrays without bit lines are stacked on top of each other in separateplanes but then share common bit lines that extend up through theplanes. These bit lines are those whose voltages or currents depend onthe data being read from or programmed into the memory.

The memory elements used in the three-dimensional array are preferablyvariable resistive memory elements. That is, the resistance (and thusinversely the conductance) of the individual memory elements istypically changed as a result of a voltage placed across theorthogonally intersecting conductors to which the element is connected.Depending on the type of variable resistive element, the state maychange in response to a voltage across it, a level of current though it,an amount of electric field across it, a level of heat applied to it,and the like. With some variable resistive element material, it is theamount of time that the voltage, current, electric field, heat and thelike is applied to the element that determines when its conductive statechanges and the direction in which the change takes place. In betweensuch state changing operations, the resistance of the memory elementremains unchanged, so is non-volatile. The three-dimensional arrayarchitecture summarized above may be implemented with a memory elementmaterial selected from a wide variety of such materials having differentproperties and operating characteristics.

The resistance of the memory element, and thus its detectable storagestate, can be repetitively set from an initial level to another leveland then re-set back to the initial level. For some materials, theamount or duration of the voltage, current, electric field, heat and thelike applied to change its state in one direction is different(asymmetrical) with that applied to change in another direction. Withtwo detectable states, each memory element stores one-bit of data. Withthe use of some materials, more than one bit of data may be stored ineach memory element by designating more than two stable levels ofresistance as detectable states of the memory element. Thethree-dimensional array architecture herein is quite versatile in theway it may be operated.

This three-dimensional architecture also allows limiting the extent andnumber of unaddressed (non-selected) resistive memory elements acrosswhich an undesired level of voltage is applied during reading andprogramming operations conducted on other addressed (selected) memoryelements. The risk of disturbing the states of unaddressed memoryelements and the levels of leakage current passing through unaddressedelements may be significantly reduced from those experienced in otherarrays using the same memory element material. Leakage currents areundesirable because they can alter the apparent currents being read fromaddressed memory elements, thereby making it difficult to accuratelyread the states of addressed (selected) memory elements. Leakagecurrents are also undesirable because they add to the overall power drawby an array and therefore undesirably causes the power supply to have tobe made larger than is desirable. Because of the relatively small extentof unaddressed memory elements that have voltages applied duringprogramming and reading of addressed memory elements, the array with thethree-dimensional architecture herein may be made to include a muchlarger number of memory elements without introducing errors in readingand exceeding reasonable power supply capabilities.

In addition, the three-dimensional architecture herein allows variableresistance memory elements to be connected at preferably orthogonalcrossings of bit and word line conductors without the need for diodes orother non-linear elements being connected in series with the variableresistive elements. In existing arrays of variable resistance memoryelements, a diode is commonly connected in series with each memoryelement in order to reduce the leakage current though the element whenit is unselected but nevertheless has a voltage difference placed acrossit, such as can occur when the unselected memory element is connected toa bit or word line carrying voltages to selected memory elementsconnected to those same lines. The absence of the need for diodessignificantly reduces the complexity of the array and thus the number ofprocessing steps required to manufacture it.

Indeed, the manufacture of the three-dimensional array of memoryelements herein is much simpler than other three-dimensional arraysusing the same type of memory elements. In particular, a fewer number ofmasks is required to form the elements of each plane of the array. Thetotal number of processing steps needed to form integrated circuits withthe three-dimensional array is thus reduced, as is the cost of theresulting integrated circuit.

Various aspects, advantages, features and details of the innovativethree-dimensional variable resistive element memory system are includedin a description of exemplary examples thereof that follows, whichdescription should be taken in conjunction with the accompanyingdrawings.

All patents, patent applications, articles, other publications,documents and things referenced herein are hereby incorporated herein bythis reference in their entirety for all purposes. To the extent of anyinconsistency or conflict in the definition or use of terms between anyof the incorporated publications, documents or things and the presentapplication, those of the present application shall prevail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an equivalent circuit of a portion of a three-dimensionalarray of variable resistance memory elements, wherein the array hasvertical bit lines;

FIG. 2 is a schematic block diagram of a re-programmable non-volatilememory system which utilizes the memory cell array of FIG. 1, and whichindicates connection of the memory system with a host system;

FIG. 3 provides plan views of the two planes and substrate of thethree-dimensional array of FIG. 1, with some structure added;

FIG. 4 is an expanded view of a portion of one of the planes of FIG. 3,annotated to show effects of programming data therein;

FIG. 5 is an expanded view of a portion of one of the planes of FIG. 3,annotated to show effects of reading data therefrom;

FIG. 6 illustrates an example memory storage element;

FIG. 7 is an isometric view of a portion of the three-dimensional arrayshown in FIG. 1 according to a first specific example of animplementation thereof;

FIG. 8 is cross-section of a portion of the three-dimensional arrayshown in FIG. 1 according to a second specific example of animplementation thereof;

FIGS. 9-14 illustrate a process of forming the three-dimensional arrayexample of FIG. 8; and

FIG. 15 is a cross-section of a portion of the three-dimensional arrayshown in FIG. 1 according to a third specific example of animplementation thereof.

FIG. 16 illustrates the read bias voltages and current leakage acrossmultiple planes of the 3D memory shown in FIG. 1 and FIG. 3.

FIG. 17 illustrates a three-dimensional memory with adouble-global-bit-line architecture for improved access to a set oflocal bit lines.

FIG. 18 illustrates the elimination of leakage currents in thedouble-global-line architecture 3D array of FIG. 17.

FIG. 19 illustrates schematically a single-sided word line architecture.

FIG. 20 illustrates one plane and substrate of the 3D array with thesingle-sided word line architecture.

FIG. 21 illustrates the elimination of leakage currents in thesingle-sided word-line architecture 3-D array of FIGS. 19 and 20.

FIG. 22 is an isometric view of a portion of the 3D array with thesingle-sided word line architecture shown in FIG. 19.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring initially to FIG. 1, an architecture of a three-dimensionalmemory 10 is schematically and generally illustrated in the form of anequivalent circuit of a portion of such a memory. This is a specificexample of the three-dimensional array summarized above. A standardthree-dimensional rectangular coordinate system 11 is used forreference, the directions of each of vectors x, y and z being orthogonalwith the other two.

A circuit for selectively connecting internal memory elements withexternal data circuits is preferably formed in a semiconductor substrate13. In this specific example, a two-dimensional array of select orswitching devices Q_(xy) are utilized, where x gives a relative positionof the device in the x-direction and y its relative position in they-direction. The individual devices Q, may be a select gate or selecttransistor, as examples. Global bit lines (GBL_(x)) are elongated in they-direction and have relative positions in the x-direction that areindicated by the subscript. The global bit lines (GBL_(x)) areindividually connectable with the source or drain of the select devicesQ having the same position in the x-direction, although during readingand also typically programming only one select device connected with aspecific global bit line is turned on at time. The other of the sourceor drain of the individual select devices Q is connected with one of thelocal bit lines (LBL_(xy)). The local bit lines are elongatedvertically, in the z-direction, and form a regular two-dimensional arrayin the x (row) and y (column) directions.

In order to connect one set (in this example, designated as one row) oflocal bit lines with corresponding global bit lines, control gate linesSG_(y) are elongated in the x-direction and connect with controlterminals (gates) of a single row of select devices Q_(xy) having acommon position in the y-direction. The select devices Q_(xy) thereforeconnect one row of local bit lines (LBL_(xy)) across the x-direction(having the same position in the y-direction) at a time to correspondingones of the global bit-lines (GBL_(x)), depending upon which of thecontrol gate lines SG_(y) receives a voltage that turns on the selectdevices to which it is connected. The remaining control gate linesreceive voltages that keep their connected select devices off. It may benoted that since only one select device (Q_(xy)) is used with each ofthe local bit lines (LBL_(xy)), the pitch of the array across thesemiconductor substrate in both x and y-directions may be made verysmall, and thus the density of the memory storage elements large.

Memory storage elements M_(zxy) are formed in a plurality of planespositioned at different distances in the z-direction above the substrate13. Two planes 1 and 2 are illustrated in FIG. 1 but there willtypically be more, such as 4, 6 or even more. In each plane at distancez, word lines WL_(zy) are elongated in the x-direction and spaced apartin the y-direction between the local bit-lines (LBL_(xy)). The wordlines WL_(zy) of each plane individually cross adjacent two of the localbit-lines LBL_(xy) on either side of the word lines. The individualmemory storage elements M_(zxy) are connected between one local bit lineLBL_(xy) and one word line WL_(zy) adjacent these individual crossings.An individual memory element M_(zxy) is therefore addressable by placingproper voltages on the local bit line LBL_(xy) and word line WL_(zy)between which the memory element is connected. The voltages are chosento provide the electrical stimulus necessary to cause the state of thememory element to change from an existing state to the desired newstate. The levels, duration and other characteristics of these voltagesdepend upon the material that is used for the memory elements.

Each “plane” of the three-dimensional memory cell structure is typicallyformed of at least two layers, one in which the conductive word linesWL_(zy) are positioned and another of a dielectric material thatelectrically isolates the planes from each other. Additional layers mayalso be present in each plane, depending for example on the structure ofthe memory elements M_(zxy). The planes are stacked on top of each otheron a semiconductor substrate with the local bit lines LBL_(xy) beingconnected with storage elements M_(zxy) of each plane through which thelocal bit lines extend.

FIG. 2 is a block diagram of an illustrative memory system that can usethe three-dimensional memory 10 of FIG. 1. Data input-output circuits 21are connected to provide (during programming) and receive (duringreading) analog electrical quantities in parallel over the globalbit-lines GBL_(x) of FIG. 1 that are representative of data stored inaddressed storage elements M_(zxy). The circuits 21 typically containsense amplifiers for converting these electrical quantities into digitaldata values during reading, which digital values are then conveyed overlines 23 to a memory system controller 25. Conversely, data to beprogrammed into the array 10 are sent by the controller 25 to theinput-output circuits 21, which then programs that data into addressedmemory element by placing proper voltages on the global bit linesGBL_(x). For binary operation, one voltage level is typically placed ona global bit line to represent a binary “1” and another voltage level torepresent a binary “0”. The memory elements are addressed for reading orprogramming by voltages placed on the word lines WL_(zy) and select gatecontrol lines SG_(y) by respective word line select circuits 27 andlocal bit line circuits 29. In the specific three-dimensional array ofFIG. 1, the memory elements lying between a selected word line and anyof the local bit lines LBL_(xy) connected at one instance through theselect devices Q_(xy) to the global bit lines GBL_(x) may be addressedfor programming or reading by appropriate voltages being applied throughthe select circuits 27 and 29.

The memory system controller 25 typically receives data from and sendsdata to a host system 31. The controller 25 usually contains an amountof random-access-memory (RAM) 34 for temporarily storing such data andoperating information. Commands, status signals and addresses of databeing read or programmed are also exchanged between the controller 25and host 31. The memory system operates with a wide variety of hostsystems. They include personal computers (PCs), laptop and otherportable computers, cellular telephones, personal digital assistants(PDAs), digital still cameras, digital movie cameras and portable audioplayers. The host typically includes a built-in receptacle 33 for one ormore types of memory cards or flash drives that accepts a mating memorysystem plug 35 of the memory system but some hosts require the use ofadapters into which a memory card is plugged, and others require the useof cables therebetween. Alternatively, the memory system may be builtinto the host system as an integral part thereof.

The memory system controller 25 conveys to decoder/driver circuits 37commands received from the host. Similarly, status signals generated bythe memory system are communicated to the controller 25 from thecircuits 37. The circuits 37 can be simple logic circuits in the casewhere the controller controls nearly all of the memory operations, orcan include a state machine to control at least some of the repetitivememory operations necessary to carry out given commands. Control signalsresulting from decoding commands are applied from the circuits 37 to theword line select circuits 27, local bit line select circuits 29 and datainput-output circuits 21. Also connected to the circuits 27 and 29 areaddress lines 39 from the controller that carry physical addresses ofmemory elements to be accessed within the array 10 in order to carry outa command from the host. The physical addresses correspond to logicaladdresses received from the host system 31, the conversion being made bythe controller 25 and/or the decoder/driver 37. As a result, thecircuits 29 partially address the designated storage elements within thearray 10 by placing proper voltages on the control elements of theselect devices Q_(xy) to connect selected local bit lines (LBL_(xy))with the global bit lines (GBL_(x)). The addressing is completed by thecircuits 27 applying proper voltages to the word lines WL_(zy) of thearray.

Although the memory system of FIG. 2 utilizes the three-dimensionalmemory element array 10 of FIG. 1, the system is not limited to use ofonly that array architecture. A given memory system may alternativelycombine this type of memory with other another type including flashmemory, such as flash having a NAND memory cell array architecture, amagnetic disk drive or some other type of memory. The other type ofmemory may have its own controller or may in some cases share thecontroller 25 with the three-dimensional memory cell array 10,particularly if there is some compatibility between the two types ofmemory at an operational level.

Although each of the memory elements M_(zxy) in the array of FIG. 1 maybe individually addressed for changing its state according to incomingdata or for reading its existing storage state, it is certainlypreferable to program and read the array in units of multiple memoryelements in parallel. In the three-dimensional array of FIG. 1, one rowof memory elements on one plane may be programmed and read in parallel.The number of memory elements operated in parallel depends on the numberof memory elements connected to the selected word line. In some arrays,the word lines may be segmented (not shown in FIG. 1) so that only aportion of the total number of memory elements connected along theirlength may be addressed for parallel operation, namely the memoryelements connected to a selected one of the segments.

Previously programmed memory elements whose data have become obsoletemay be addressed and re-programmed from the states in which they werepreviously programmed. The states of the memory elements beingre-programmed in parallel will therefore most often have differentstarting states among them. This is acceptable for many memory elementmaterials but it is usually preferred to re-set a group of memoryelements to a common state before they are re-programmed. For thispurpose, the memory elements may be grouped into blocks, where thememory elements of each block are simultaneously reset to a commonstate, preferably one of the programmed states, in preparation forsubsequently programming them. If the memory element material being usedis characterized by changing from a first to a second state insignificantly less time than it takes to be changed from the secondstate back to the first state, then the reset operation is preferablychosen to cause the transition taking the longer time to be made. Theprogramming is then done faster than resetting. The longer reset time isusually not a problem since resetting blocks of memory elementscontaining nothing but obsolete data is typically accomplished in a highpercentage of the cases in the background, therefore not adverselyimpacting the programming performance of the memory system.

With the use of block re-setting of memory elements, a three-dimensionalarray of variable resistive memory elements may be operated in a mannersimilar to current flash memory cell arrays. Resetting a block of memoryelements to a common state corresponds to erasing a block of flashmemory cells to an erased state. The individual blocks of memoryelements herein may be further divided into a plurality of pages ofstorage elements, wherein the memory elements of a page are programmedand read together. This is like the use of pages in flash memories. Thememory elements of an individual page are programmed and read together.Of course, when programming, those memory elements that are to storedata that are represented by the reset state are not changed from thereset state. Those of the memory elements of a page that need to bechanged to another state in order to represent the data being stored inthem have their states changed by the programming operation.

An example of use of such blocks and pages is illustrated in FIG. 3,which provides plan schematic views of planes 1 and 2 of the array ofFIG. 1. The different word lines WL_(zy) that extend across each of theplanes and the local bit lines LBL_(xy) that extend through the planesare shown in two-dimensions. Individual blocks are made up of memoryelements connected to both sides of one word line, or one segment of aword line if the word lines are segmented, in a single one of theplanes. There are therefore a very large number of such blocks in eachplane of the array. In the block illustrated in FIG. 3, each of thememory elements M₁₁₄, M₁₂₄, M₁₃₄, M₁₁₅, M₁₂₅ and M₁₃₅ connected to bothsides of one word line WL₁₂ form the block. Of course, there will bemany more memory elements connected along the length of a word line butonly a few of them are illustrated, for simplicity. The memory elementsof each block are connected between the single word line and differentones of the local bit lines, namely, for the block illustrated in FIG.3, between the word line WL₁₂ and respective local bit lines LBL₁₂,LBL₂₂, LBL₃₂, LBL₁₃, LBL₂₃ and LBL₃₃.

A page is also illustrated in FIG. 3. In the specific embodiment beingdescribed, there are two pages per block. One page is formed by thememory elements along one side of the word line of the block and theother page by the memory elements along the opposite side of the wordline. The example page marked in FIG. 3 is formed by memory elementsM₁₁₄, M₁₂₄ and M₁₃₄. Of course, a page will typically have a very largenumber of memory elements in order to be able to program and read alarge amount of data at one time. Only a few of the storage elements ofthe page of FIG. 3 are included, for simplicity in explanation.

Example resetting, programming and reading operations of the memoryarray of FIGS. 1 and 3, when operated as array 10 in the memory systemof FIG. 2, will now be described. For these examples, each of the memoryelements M_(zxy) is taken to include a non-volatile memory material thatcan be switched between two stable states of different resistance levelsby impressing voltages (or currents) of different polarity across thememory element, or voltages of the same polarity but differentmagnitudes and/or duration. For example, one class of material may beplaced into a high resistance state by passing current in one directionthrough the element, and into a low resistance state by passing currentin the other direction through the element. Or, in the case of switchingusing the same voltage polarity, one element may need a higher voltageand a shorter time to switch to a high resistance state and a lowervoltage and a longer time to switch to a lower resistance state. Theseare the two memory states of the individual memory elements thatindicate storage of one bit of data, which is either a “0” or a “1”,depending upon the memory element state.

To reset (erase) a block of memory elements, the memory elements in thatblock are placed into their high resistance state. This state will bedesignated as the logical data state “1”, following the convention usedin current flash memory arrays but it could alternatively be designatedto be a “0”. As shown by the example in FIG. 3, a block includes all thememory elements that are electrically connected to one word line WL orsegment thereof. A block is the smallest unit of memory elements in thearray that are reset together. It can include thousands of memoryelements. If a row of memory elements on one side of a word lineincludes 1000 of them, for example, a block will have 2000 memoryelements from the two rows on either side of the word line.

The following steps may be taken to reset all the memory elements of ablock, using the block illustrated in FIG. 3 as an example:

-   -   1. Set all of the global bit lines (GBL₁, GBL₂ and GBL₃ in the        array of FIGS. 1 and 3) to zero volts, by the circuits 21 of        FIG. 2.    -   2. Set at least the two select gate lines on either side of the        one word line of the block to H′ volts, so that the local bit        lines on each side of the word line in the y-direction are        connected to their respective global bit lines through their        select devices and therefore brought to zero volts. The voltage        H′ is made high enough to turn on the select devices Q_(xy),        like something in a range of 1-3 volts, typically 2 volts. The        block shown in FIG. 3 includes the word line WL₁₂, so the select        gate lines SG₂ and SG₃ (FIG. 1) on either side of that word line        are set to H′ volts, by the circuits 29 of FIG. 2, in order to        turn on the select devices Q₁₂, Q₂₂, Q₃₂, Q₁₃, Q₂₃ and Q₃₃. This        causes each of the local bit lines LBL₁₂, LBL₂₂, LBL₃₂, LBL₁₃,        LBL₂₃ and LBL₃₃ in two adjacent rows extending in the        x-direction to be connected to respective ones of the global bit        lines GBL1, GBL2 and GBL3. Two of the local bit lines adjacent        to each other in the y-direction are connected to a single        global bit line. Those local bit lines are then set to the zero        volts of the global bit lines. The remaining local bit lines        preferably remain unconnected and with their voltages floating.    -   3. Set the word line of the block being reset to H volts. This        reset voltage value is dependent on the switching material in        the memory element and can be between a fraction of a volt to a        few volts. All other word lines of the array, including the        other word lines of selected plane 1 and all the word lines on        the other unselected planes, are set to zero volts. In the array        of FIGS. 1 and 3, word line WL₁₂ is placed at H volts, while all        other word lines in the array are placed at zero volts, all by        the circuits 27 of FIG. 2.

The result is that H volts are placed across each of the memory elementsof the block. In the example block of FIG. 3, this includes the memoryelements M₁₁₄, M₁₂₄, M₁₃₄, M₁₁₅, M₁₂₅ and M₁₃₅. For the type of memorymaterial being used as an example, the resulting currents through thesememory elements places any of them not already in a high resistancestate, into that re-set state.

It may be noted that no stray currents will flow because only one wordline has a non-zero voltage. The voltage on the one word line of theblock can cause current to flow to ground only through the memoryelements of the block. There is also nothing that can drive any of theunselected and electrically floating local bit lines to H volts, so novoltage difference will exist across any other memory elements of thearray outside of the block. Therefore no voltages are applied acrossunselected memory elements in other blocks that can cause them to beinadvertently disturbed or reset.

It may also be noted that multiple blocks may be concurrently reset bysetting any combination of word lines and the adjacent select gates to Hor H′ respectively. In this case, the only penalty for doing so is anincrease in the amount of current that is required to simultaneouslyreset an increased number of memory elements. This affects the size ofthe power supply that is required.

The memory elements of a page are preferably programmed concurrently, inorder to increase the parallelism of the memory system operation. Anexpanded version of the page indicated in FIG. 3 is provided in FIG. 4,with annotations added to illustrate a programming operation. Theindividual memory elements of the page are initially in their resetstate because all the memory elements of its block have previously beenreset. The reset state is taken herein to represent a logical data “1”.For any of these memory elements to store a logical data “0” inaccordance with incoming data being programmed into the page, thosememory elements are switched into their low resistance state, their setstate, while the remaining memory elements of the page remain in thereset state.

For programming a page, only one row of select devices is turned on,resulting in only one row of local bit lines being connected to theglobal bit lines. This connection alternatively allows the memoryelements of both pages of the block to be programmed in two sequentialprogramming cycles, which then makes the number of memory elements inthe reset and programming units equal.

Referring to FIGS. 3 and 4, an example programming operation within theindicated one page of memory elements M₁₁₄, M₁₂₄ and M₁₃₄ is described,as follows:

-   -   1. The voltages placed on the global bit lines are in accordance        with the pattern of data received by the memory system for        programming. In the example of FIG. 4, GBL₁ carries logical data        bit “1”, GBL₂ the logical bit “0” and GBL₃ the logical bit “1.”        The bit lines are set respectively to corresponding voltages M,        H and M, as shown, where the M level voltage is high but not        sufficient to program a memory element and the H level is high        enough to force a memory element into the programmed state. The        M level voltage may be about one-half of the H level voltage,        between zero volts and H. For example, a M level can be 0.7        volt, and a H level can be 1.5 volt. The H level used for        programming is not necessary the same as the H level used for        resetting or reading. In this case, according to the received        data, memory elements M₁₁₄ and M₁₃₄ are to remain in their reset        state, while memory element M₁₂₄ is being programmed. Therefore,        the programming voltages are applied only to memory element M₁₂₄        of this page by the following steps.    -   2. Set the word line of the page being programmed to 0 volts, in        this case selected word line WL₁₂. This is the only word line to        which the memory elements of the page are connected. Each of the        other word lines on all planes is set to the M level. These word        line voltages are applied by the circuits 27 of FIG. 2.    -   3. Set one of the select gate lines below and on either side of        the selected word line to the H′ voltage level, in order to        select a page for programming. For the page indicated in FIGS. 3        and 4, the H′ voltage is placed on select gate line SG₂ in order        to turn on select devices Q₁₂, Q₂₂ and Q₃₂ (FIG. 1). All other        select gate lines, namely lines SG₁ and SG₃ in this example, are        set to 0 volts in order to keep their select devices off. The        select gate line voltages are applied by the circuits 29 of        FIG. 2. This connects one row of local bit lines to the global        bit lines and leaves all other local bit lines floating. In this        example, the row of local bit lines LBL₁₂, LBL₂₂ and LBL₃₂ are        connected to the respective global bit lines GBL₁, GBL₂ and GBL₃        through the select devices that are turned on, while all other        local bit lines (LBLs) of the array are left floating.

The result of this operation, for the example memory element materialmentioned above, is that a programming current I_(prog) is sent throughthe memory element M₁₂₄, thereby causing that memory element to changefrom a reset to a set (programmed) state. The same will occur with othermemory elements (not shown) that are connected between the selected wordline WL₁₂ and a local bit line (LBL) that has the programming voltagelevel H applied.

An example of the relative timing of applying the above-listedprogramming voltages is to initially set all the global bit lines(GBLs), the selected select gate line (SG), the selected word line andtwo adjacent word lines on either side of the selected word line on theone page all to the voltage level M. After this, selected ones of theGBLs are raised to the voltage level H according to the data beingprogrammed while simultaneously dropping the voltage of the selectedword line to 0 volts for the duration of the programming cycle. The wordlines in plane 1 other than the selected word line WL₁₂ and all wordlines in the unselected other planes can be weakly driven to M, somelower voltage or allowed to float in order to reduce power that must bedelivered by word line drivers that are part of the circuits 27 of FIG.2.

By floating all the local bit lines other than the selected row (in thisexample, all but LBL₁₂, LBL₂₂ and LBL₃₂), voltages can be looselycoupled to outer word lines of the selected plane 1 and word lines ofother planes that are allowed to float through memory elements in theirlow resistance state (programmed) that are connected between thefloating local bit lines and adjacent word lines. These outer word linesof the selected plane and word lines in unselected planes, althoughallowed to float, may eventually be driven up to voltage level M througha combination of programmed memory elements.

There are typically parasitic currents present during the programmingoperation that can increase the currents that must be supplied throughthe selected word line and global bit lines. During programming thereare two sources of parasitic currents, one to the adjacent page in adifferent block and another to the adjacent page in the same block. Anexample of the first is the parasitic current I_(P1) shown on FIG. 4from the local bit line LBL₂₂ that has been raised to the voltage levelH during programming. The memory element M₁₂₃ is connected between thatvoltage and the voltage level M on its word line WL₁₁. This voltagedifference can cause the parasitic current −I_(P1) to flow. Since thereis no such voltage difference between the local bit lines LBL₁₂ or LBL₃₂and the word line WL₁₁, no such parasitic current flows through eitherof the memory elements M₁₁₃ or M₁₃₃, a result of these memory elementsremaining in the reset state according to the data being programmed.

Other parasitic currents can similarly flow from the same local bit lineLBL₂₂ to an adjacent word line in other planes. The presence of thesecurrents may limit the number of planes that can be included in thememory system since the total current may increase with the number ofplanes. The limitation for programming is in the current capacity of thememory power supply, so the maximum number of planes is a tradeoffbetween the size of the power supply and the number of planes. A numberof 4-8 planes may generally be used in most cases.

The other source of parasitic currents during programming is to anadjacent page in the same block. The local bit lines that are leftfloating (all but those connected to the row of memory elements beingprogrammed) will tend to be driven to the voltage level M of unselectedword lines through any programmed memory element on any plane. This inturn can cause parasitic currents to flow in the selected plane fromthese local bit lines at the M voltage level to the selected word linethat is at zero volts. An example of this is given by the currentsI_(P2), I_(P3) and I_(P4) shown in FIG. 4. In general, these currentswill be much less than the other parasitic current I_(P1) discussedabove, since these currents flow only through those memory elements intheir conductive state that are adjacent to the selected word line inthe selected plane.

The above-described programming techniques ensure that the selected pageis programmed (local bit lines at H, selected word line at 0) and thatadjacent unselected word lines are at M. As mentioned earlier, otherunselected word lines can be weakly driven to M or initially driven to Mand then left floating. Alternately, word lines in any plane distantfrom the selected word line (for example, more than 5 word lines away)can also be left uncharged (at ground) or floating because the parasiticcurrents flowing to them are so low as to be negligible compared to theidentified parasitic currents since they must flow through a seriescombination of five or more ON devices (devices in their low resistancestate). This can reduce the power dissipation caused by charging a largenumber of word lines.

While the above description assumes that each memory element of the pagebeing programmed will reach its desired ON value with one application ofa programming pulse, a program-verify technique commonly used in NOR orNAND flash memory technology may alternately be used. In this process, acomplete programming operation for a given page includes of a series ofindividual programming operations in which a smaller change in ONresistance occurs within each program operation. Interspersed betweeneach program operation is a verify (read) operation that determineswhether an individual memory element has reached its desired programmedlevel of resistance or conductance consistent with the data beingprogrammed in the memory element. The sequence of program/verify isterminated for each memory element as it is verified to reach thedesired value of resistance or conductance. After all of memory elementsbeing programmed are verified to have reached their desired programmedvalue, programming of the page of memory elements is then completed. Anexample of this technique is described in U.S. Pat. No. 5,172,338.

With reference primarily to FIG. 5, the parallel reading of the statesof a page of memory elements, such as the memory elements M₁₁₄, M₁₂₄ andM₁₃₄, is described. The steps of an example reading process are asfollows:

-   -   1. Set all the global bit lines GBLs and all the word lines WL        to a voltage V_(R). The voltage V_(R) is simply a convenient        reference voltage and can be any number of values but will        typically be between 0 and 1 volt. In general, for operating        modes where repeated reads occur, it is convenient to set all        word lines in the array to V_(R) in order to reduce parasitic        read currents, even though this requires charging all the word        lines. However, as an alternative, it is only necessary to raise        the selected word line (WL₁₂ in FIG. 5), the word line in each        of the other planes that is in the same position as the selected        word line and the immediately adjacent word lines in all planes        to V_(R).    -   2. Turn on one row of select devices by placing a voltage on the        control line adjacent to the selected word line in order to        define the page to be read. In the example of FIGS. 1 and 5, a        voltage is applied to the control line SG₂ in order to turn on        the select devices Q₁₂, Q₂₂ and Q₃₂. This connects one row of        local bit lines LBL₁₂, LBL₂₂ and LBL₃₂ to their respective        global bit lines GBL₁, GBL₂ and GBL₃. These local bit lines are        then connected to individual sense amplifiers (SA) that are        present in the circuits 21 of FIG. 2, and assume the potential        V_(R) of the global bit lines to which they are connected. All        other local bit lines LBLs are allowed to float.    -   3. Set the selected word line (WL₁₂) to a voltage of        V_(R)±Vsense. The sign of Vsense is chosen based on the sense        amplifier and has a magnitude of about 0.5 volt. The voltages on        all other word lines remain the same.    -   4. Sense current flowing into (V_(R)+Vsense) or out of        (V_(R)−Vsense) each sense amplifier for time T. These are the        currents I_(R1), I_(R2) and I_(R3) shown to be flowing through        the addressed memory elements of the example of FIG. 5, which        are proportional to the programmed states of the respective        memory elements M₁₁₄, M₁₂₄ and M₁₃₄. The states of the memory        elements M₁₁₄, M₁₂₄ and M₁₃₄ are then given by binary outputs of        the sense amplifiers within the circuits 21 that are connected        to the respective global bit lines GBL₁, GBL₂ and GBL₃. These        sense amplifier outputs are then sent over the lines 23 (FIG. 2)        to the controller 25, which then provides the read data to the        host 31.    -   5. Turn off the select devices (Q₁₂, Q₂₂ and Q₃₂) by removing        the voltage from the select gate line (SG₂), in order to        disconnect the local bit lines from the global bit lines, and        return the selected word line (WL₁₂) to the voltage V_(R).

Parasitic currents during such a read operation have two undesirableeffects. As with programming, parasitic currents place increased demandson the memory system power supply. In addition, it is possible forparasitic currents to exist that are erroneously included in thecurrents though the addressed memory elements that are being read. Thiscan therefore lead to erroneous read results if such parasitic currentsare large enough.

As in the programming case, all of the local bit lines except theselected row (LBL₁₂, LBL₂₂ and LBL₃₂ in the example of FIG. 5) arefloating. But the potential of the floating local bit lines may bedriven to V_(R) by any memory element that is in its programmed (lowresistance) state and connected between a floating local bit line and aword line at V_(R), in any plane. A parasitic current comparable toI_(P1) in the programming case (FIG. 4) is not present during data readbecause both the selected local bit lines and the adjacent non-selectedword lines are both at V_(R). Parasitic currents may flow, however,through low resistance memory elements connected between floating localbit lines and the selected word line. These are comparable to thecurrents I_(P2), I_(P3), and I_(P4) during programming (FIG. 4),indicated as I_(P5), I_(P6) and I_(P7) in FIG. 5. Each of these currentscan be equal in magnitude to the maximum read current through anaddressed memory element. However, these parasitic currents are flowingfrom the word lines at the voltage V_(R) to the selected word line at avoltage V_(R)±Vsense without flowing through the sense amplifiers. Theseparasitic currents will not flow through the selected local bit lines(LBL₁₂, LBL₂₂ and LBL₃₂ in FIG. 5) to which the sense amplifiers areconnected. Although they contribute to power dissipation, theseparasitic currents do not therefore introduce a sensing error.

Although the neighboring word lines should be at V_(R) to minimizeparasitic currents, as in the programming case it may be desirable toweakly drive these word lines or even allow them to float. In onevariation, the selected word line and the neighboring word lines can bepre-charged to V_(R) and then allowed to float. When the sense amplifieris energized, it may charge them to V_(R) so that the potential on theselines is accurately set by the reference voltage from the senseamplifier (as opposed to the reference voltage from the word linedriver). This can occur before the selected word line is changed toV_(R)±Vsense but the sense amplifier current is not measured until thischarging transient is completed.

Reference cells may also be included within the memory array 10 tofacilitate any or all of the common data operations (erase, program, orread). A reference cell is a cell that is structurally as nearlyidentical to a data cell as possible in which the resistance is set to aparticular value. They are useful to cancel or track resistance drift ofdata cells associated with temperature, process non-uniformities,repeated programming, time or other cell properties that may vary duringoperation of the memory. Typically they are set to have a resistanceabove the highest acceptable low resistance value of a memory element inone data state (such as the ON resistance) and below the lowestacceptable high resistance value of a memory element in another datastate (such as the OFF resistance). Reference cells may be “global” to aplane or the entire array, or may be contained within each block orpage.

In one embodiment, multiple reference cells may be contained within eachpage. The number of such cells may be only a few (less than 10), or maybe up to a several percent of the total number of cells within eachpage. In this case, the reference cells are typically reset and writtenin a separate operation independent of the data within the page. Forexample, they may be set one time in the factory, or they may be setonce or multiple times during operation of the memory array. During areset operation described above, all of the global bit lines are setlow, but this can be modified to only set the global bit linesassociated with the memory elements being reset to a low value while theglobal bit lines associated with the reference cells are set to anintermediate value, thus inhibiting them from being reset. Alternately,to reset reference cells within a given block, the global bit linesassociated with the reference cells are set to a low value while theglobal bit lines associated with the data cells are set to anintermediate value. During programming, this process is reversed and theglobal bit lines associated with the reference cells are raised to ahigh value to set the reference cells to a desired ON resistance whilethe memory elements remain in the reset state. Typically the programmingvoltages or times will be changed to program reference cells to a higherON resistance than when programming memory elements.

If, for example, the number of reference cells in each page is chosen tobe 1% of the number of data storage memory elements, then they may bephysically arranged along each word line such that each reference cellis separated from its neighbor by 100 data cells, and the senseamplifier associated with reading the reference cell can share itsreference information with the intervening sense amplifiers readingdata. Reference cells can be used during programming to ensure the datais programmed with sufficient margin. Further information regarding theuse of reference cells within a page can be found in U.S. Pat. Nos.6,222,762, 6,538,922, 6,678,192 and 7,237,074.

In a particular embodiment, reference cells may be used to approximatelycancel parasitic currents in the array. In this case the value of theresistance of the reference cell(s) is set to that of the reset staterather than a value between the reset state and a data state asdescribed earlier. The current in each reference cell can be measured byits associated sense amplifier and this current subtracted fromneighboring data cells. In this case, the reference cell isapproximating the parasitic currents flowing in a region of the memoryarray that tracks and is similar to the parasitic currents flowing inthat region of the array during a data operation. This correction can beapplied in a two-step operation (measure the parasitic current in thereference cells and subsequently subtract its value from that obtainedduring a data operation) or simultaneously with the data operation. Oneway in which simultaneous operation is possible is to use the referencecell to adjust the timing or reference levels of the adjacent data senseamplifiers. An example of this is shown in U.S. Pat. No. 7,324,393.

In conventional two-dimensional arrays of variable resistance memoryelements, a diode is usually included in series with the memory elementbetween the crossing bit and word lines. The primary purpose of thediodes is to reduce the number and magnitudes of parasitic currentsduring resetting (erasing), programming and reading the memory elements.A significant advantage of the three-dimensional array herein is thatresulting parasitic currents are fewer and therefore have a reducednegative effect on operation of the array than in other types of arrays.

Diodes may also be connected in series with the individual memoryelements of the three-dimensional array, as currently done in otherarrays of variable resistive memory elements, in order to reduce furtherthe number of parasitic currents but there are disadvantages in doingso. Primarily, the manufacturing process becomes more complicated. Addedmasks and added manufacturing steps are then necessary. Also, sinceformation of the silicon p-n diodes often requires at least one hightemperature step, the word lines and local bit lines cannot then be madeof metal having a low melting point, such as aluminum that is commonlyused in integrated circuit manufacturing, because it may melt during thesubsequent high temperature step. Use of a metal, or composite materialincluding a metal, is preferred because of its higher conductivity thanthe conductively doped polysilicon material that is typically used forbit and word lines because of being exposed to such high temperatures.An example of an array of resistive switching memory elements having adiode formed as part of the individual memory elements is given inpatent application publication no. US 2009/0001344 A1.

Because of the reduced number of parasitic currents in thethree-dimensional array herein, the total magnitude of parasiticcurrents can be managed without the use of such diodes. In addition tothe simpler manufacturing processes, the absence of the diodes allowsbi-polar operation; that is, an operation in which the voltage polarityto switch the memory element from its first state to its second memorystate is opposite of the voltage polarity to switch the memory elementfrom its second to its first memory state. The advantage of the bi-polaroperation over a unipolar operation (same polarity voltage is used toswitch the memory element from its first to second memory state as fromits second to first memory state) is the reduction of power to switchthe memory element and an improvement in the reliability of the memoryelement. These advantages of the bi-polar operation are seen in memoryelements in which formation and destruction of a conductive filament isthe physical mechanism for switching, as in the memory elements madefrom metal oxides and solid electrolyte materials.

The level of parasitic currents increases with the number of planes andwith the number of memory elements connected along the individual wordlines within each plane. But since the number of word lines on eachplane does not significantly affect the amount of parasitic current, theplanes may individually include a large number of word lines. Theparasitic currents resulting from a large number of memory elementsconnected along the length of individual word lines can further bemanaged by segmenting the word lines into sections of fewer numbers ofmemory elements. Erasing, programming and reading operations are thenperformed on the memory elements connected along one segment of eachword line instead of the total number of memory elements connected alongthe entire length of the word line.

The re-programmable non-volatile memory array being described herein hasmany advantages. The quantity of digital data that may be stored perunit of semiconductor substrate area is high. It may be manufacturedwith a lower cost per stored bit of data. Only a few masks are necessaryfor the entire stack of planes, rather than requiring a separate set ofmasks for each plane. The number of local bit line connections with thesubstrate is significantly reduced over other multi-plane structuresthat do not use the vertical local bit lines. The architectureeliminates the need for each memory cell to have a diode in series withthe resistive memory element, thereby further simplifying themanufacturing process and enabling the use of metal conductive lines.Also, the voltages necessary to operate the array are much lower thanthose used in current commercial flash memories.

Since at least one-half of each current path is vertical, the voltagedrops present in large cross-point arrays are significantly reduced. Thereduced length of the current path due to the shorter vertical componentmeans that there are approximately one-half the number memory cells oneach current path and thus the leakage currents are reduced as is thenumber of unselected cells disturbed during a data programming or readoperation. For example, if there are N cells associated with a word lineand N cells associated with a bit line of equal length in a conventionalarray, there are 2N cells associated or “touched” with every dataoperation. In the vertical local bit line architecture described herein,there are n cells associated with the bit line (n is the number ofplanes and is typically a small number such as 4 to 8), or N+n cells areassociated with a data operation. For a large N this means that thenumber of cells affected by a data operation is approximately one-halfas many as in a conventional three-dimensional array.

Materials Useful for the Memory Storage Elements

The material used for the non-volatile memory storage elements M_(zxy)in the array of FIG. 1 can be a chalcogenide, a metal oxide, or any oneof a number of materials that exhibit a stable, reversible shift inresistance in response to an external voltage applied to or currentpassed through the material.

Metal oxides are characterized by being insulating when initiallydeposited. One suitable metal oxide is a titanium oxide (TiO_(x)). Apreviously reported memory element using this material is illustrated inFIG. 6. In this case, near-stoichiometric TiO₂ bulk material is alteredin an annealing process to create an oxygen deficient layer (or a layerwith oxygen vacancies) in proximity of the bottom electrode. The topplatinum electrode, with its high work function, creates a highpotential Pt/TiO₂ barrier for electrons. As a result, at moderatevoltages (below one volt), a very low current will flow through thestructure. The bottom Pt/TiO_(2-x) barrier is lowered by the presence ofthe oxygen vacancies (O⁺ ₂) and behaves as a low resistance contact(ohmic contact). (The oxygen vacancies in TiO₂ are known to act asn-type dopant, transforming the insulating oxide in an electricallyconductive doped semiconductor.) The resulting composite structure is ina non-conductive (high resistance) state.

But when a large negative voltage (such as 1.5 volt) is applied acrossthe structure, the oxygen vacancies drift toward the top electrode and,as a result, the potential barrier Pt/TiO₂ is reduced and a relativelyhigh current can flow through the structure. The device is then in itslow resistance (conductive) state. Experiments reported by others haveshown that conduction is occurring in filament-like regions of the TiO₂,perhaps along grain boundaries.

The conductive path is broken by applying a large positive voltageacross the structure of FIG. 6. Under this positive bias, the oxygenvacancies move away from the proximity of the top Pt/TiO₂ barrier, and“break” the filament. The device returns to its high resistance state.Both of the conductive and non-conductive states are non-volatile.Sensing the conduction of the memory storage element by applying avoltage around 0.5 volts can easily determine the state of the memoryelement.

While this specific conduction mechanism may not apply to all metaloxides, as a group, they have a similar behavior: transition from a lowconductive state to a high conductive occurs state when appropriatevoltages are applied, and the two states are non-volatile. Examples ofother materials include HfOx, ZrOx, WOx, NiOx, CoOx, CoalOx, MnOx,ZnMn₂O₄, ZnOx, TaOx, NbOx, HfSiOx, HfAlOx. Suitable top electrodesinclude metals with a high work function (typically >4.5 eV) capable togetter oxygen in contact with the metal oxide to create oxygen vacanciesat the contact. Some examples are TaCN, TiCN, Ru, RuO, Pt, Ti rich TiOx,TiAlN, TaAlN, TiSiN, TaSiN, IrO₂. Suitable materials for the bottomelectrode are any conducting oxygen rich material such as Ti(O)N,Ta(O)N, TiN and TaN. The thicknesses of the electrodes are typically 1nm or greater. Thicknesses of the metal oxide are generally in the rangeof 5 nm to 50 nm.

Another class of materials suitable for the memory storage elements issolid electrolytes but since they are electrically conductive whendeposited, individual memory elements need to be formed and isolatedfrom one another. Solid electrolytes are somewhat similar to the metaloxides, and the conduction mechanism is assumed to be the formation of ametallic filament between the top and bottom electrode. In thisstructure the filament is formed by dissolving ions from one electrode(the oxidizable electrode) into the body of the cell (the solidelectrolyte). In one example, the solid electrolyte contains silver ionsor copper ions, and the oxidizable electrode is preferably a metalintercalated in a transition metal sulfide or selenide material such asA_(x)(MB2)_(1-x), where A is Ag or Cu, B is S or Se, and M is atransition metal such as Ta, V, or Ti, and x ranges from about 0.1 toabout 0.7. Such a composition minimizes oxidizing unwanted material intothe solid electrolyte. One example of such a composition isAg_(x)(TaS2)_(1-x). Alternate composition materials include α-AgI. Theother electrode (the indifferent or neutral electrode) should be a goodelectrical conductor while remaining insoluble in the solid electrolytematerial. Examples include metals and compounds such as W, Ni, Mo, Pt,metal silicides, and the like.

Examples of solid electrolytes materials are: TaO, GeSe or GeS. Othersystems suitable for use as solid electrolyte cells are: Cu/TaO/W,Ag/GeSe/W, Cu/GeSe/W, Cu/GeS/W, and Ag/GeS/W, where the first materialis the oxidizable electrode, the middle material is the solidelectrolyte, and the third material is the indifferent (neutral)electrode. Typical thicknesses of the solid electrolyte are between 30nm and 100 nm.

In recent years, carbon has been extensively studied as a non-volatilememory material. As a non-volatile memory element, carbon is usuallyused in two forms, conductive (or graphene like-carbon) and insulating(or amorphous carbon). The difference in the two types of carbonmaterial is the content of the carbon chemical bonds, so called sp² andsp³ hybridizations. In the sp³ configuration, the carbon valenceelectrons are kept in strong covalent bonds and as a result the sp³hybridization is non-conductive. Carbon films in which the sp³configuration dominates, are commonly referred to astetrahedral-amorphous carbon, or diamond like. In the sp² configuration,not all the carbon valence electrons are kept in covalent bonds. Theweak tight electrons (phi bonds) contribute to the electrical conductionmaking the mostly sp² configuration a conductive carbon material. Theoperation of the carbon resistive switching nonvolatile memories isbased on the fact that it is possible to transform the sp³ configurationto the sp² configuration by applying appropriate current (or voltage)pulses to the carbon structure. For example, when a very short (1-5 ns)high amplitude voltage pulse is applied across the material, theconductance is greatly reduced as the material sp² changes into an sp³form (“reset” state). It has been theorized that the high localtemperatures generated by this pulse causes disorder in the material andif the pulse is very short, the carbon “quenches” in an amorphous state(sp³ hybridization). On the other hand, when in the reset state,applying a lower voltage for a longer time (˜300 nsec) causes part ofthe material to change into the sp² form (“set” state). The carbonresistance switching non-volatile memory elements have a capacitor likeconfiguration where the top and bottom electrodes are made of hightemperature melting point metals like W, Pd, Pt and TaN.

There has been significant attention recently to the application ofcarbon nanotubes (CNTs) as a non-volatile memory material. A (singlewalled) carbon nanotube is a hollow cylinder of carbon, typically arolled and self-closing sheet one carbon atom thick, with a typicaldiameter of about 1-2 nm and a length hundreds of times greater. Suchnanotubes can demonstrate very high conductivity, and various proposalshave been made regarding compatibility with integrated circuitfabrication. It has been proposed to encapsulate “short” CNT's within aninert binder matrix to form a fabric of CNT's. These can be deposited ona silicon wafer using a spin-on or spray coating, and as applied theCNT's have a random orientation with respect to each other. When anelectric field is applied across this fabric, the CNT's tend to flex oralign themselves such that the conductivity of the fabric is changed.The switching mechanism from low-to-high resistance and the opposite isnot well understood. As in the other carbon based resistive switchingnon-volatile memories, the CNT based memories have capacitor-likeconfigurations with top and bottom electrodes made of high melting pointmetals such as those mentioned above.

Yet another class of materials suitable for the memory storage elementsis phase-change materials. A preferred group of phase-change materialsincludes chalcogenide glasses, often of a compositionGe_(x)Sb_(y)Te_(z), where preferably x=2, y=2 and z=5. GeSb has alsobeen found to be useful. Other materials include AgInSbTe, GeTe, GaSb,BaSbTe, InSbTe and various other combinations of these basic elements.Thicknesses are generally in the range of 1 nm to 500 nm. The generallyaccepted explanation for the switching mechanism is that when a highenergy pulse is applied for a very short time to cause a region of thematerial to melt, the material “quenches” in an amorphous state, whichis a low conductive state. When a lower energy pulse is applied for alonger time such that the temperature remains above the crystallizationtemperature but below the melting temperature, the material crystallizesto form poly-crystal phases of high conductivity. These devices areoften fabricated using sub-lithographic pillars, integrated with heaterelectrodes. Often the localized region undergoing the phase change maybe designed to correspond to a transition over a step edge, or a regionwhere the material crosses over a slot etched in a low thermalconductivity material. The contacting electrodes may be any high meltingmetal such as TiN, W, WN and TaN in thicknesses from 1 nm to 500 nm.

It will be noted that the memory materials in most of the foregoingexamples utilize electrodes on either side thereof whose compositionsare specifically selected. In embodiments of the three-dimensionalmemory array herein where the word lines (WL) and/or local bit lines(LBL) also form these electrodes by direct contact with the memorymaterial, those lines are preferably made of the conductive materialsdescribed above. In embodiments using additional conductive segments forat least one of the two memory element electrodes, those segments aretherefore made of the materials described above for the memory elementelectrodes.

Steering elements are commonly incorporated into controllable resistancetypes of memory storage elements. Steering elements can be a transistoror a diode. Although an advantage of the three-dimensional architecturedescribed herein is that such steering elements are not necessary, theremay be specific configurations where it is desirable to include steeringelements. The diode can be a p-n junction (not necessarily of silicon),a metal/insulator/insulator/metal (MIIM), or a Schottky typemetal/semiconductor contact but can alternately be a solid electrolyteelement. A characteristic of this type of diode is that for correctoperation in a memory array, it is necessary to be switched “on” and“off” during each address operation. Until the memory element isaddressed, the diode is in the high resistance state (“off” state) and“shields” the resistive memory element from disturb voltages. To accessa resistive memory element, three different operations are needed: a)convert the diode from high resistance to low resistance, b) program,read, or reset (erase) the memory element by application of appropriatevoltages across or currents through the diode, and c) reset (erase) thediode. In some embodiments one or more of these operations can becombined into the same step. Resetting the diode may be accomplished byapplying a reverse voltage to the memory element including a diode,which causes the diode filament to collapse and the diode to return tothe high resistance state.

For simplicity the above description has consider the simplest case ofstoring one data value within each cell: each cell is either reset orset and holds one bit of data. However, the techniques of the presentapplication are not limited to this simple case. By using various valuesof ON resistance and designing the sense amplifiers to be able todiscriminate between several of such values, each memory element canhold multiple-bits of data in a multiple-level cell (MLC). Theprinciples of such operation are described in U.S. Pat. No. 5,172,338referenced earlier. Examples of MLC technology applied to threedimensional arrays of memory elements include an article entitled“Multi-bit Memory Using Programmable Metallization Cell Technology” byKozicki et al., Proceedings of the International Conference onElectronic Devices and Memory, Grenoble, France, Jun. 12-17, 2005, pp.48-53 and “Time Discrete Voltage Sensing and Iterative ProgrammingControl for a 4F2 Multilevel CBRAM” by Schrogmeier et al. (2007Symposium on VLSI Circuits).

Specific Structural Examples of the Three-Dimensional Array

Three alternative semiconductor structures for implementing thethree-dimensional memory element array of FIG. 1 are now described.

A first example, illustrated in FIG. 7, is configured for use of memoryelement (NVM) material that is non-conductive when first deposited. Ametal oxide of the type discussed above has this characteristic. Asexplained with respect to FIG. 6, conductive filaments are formedbetween electrodes on opposite sides of the material in response toappropriate voltages placed on those electrodes. These electrodes are abit line and a word line in the array. Since the material is otherwisenon-conductive, there is no necessity to isolate the memory elements atthe cross-points of the word and bit lines from each other. Severalmemory elements may be implemented by a single continuous layer ofmaterial, which in the case of FIG. 7 are strips of NVM materialoriented vertically along opposite sides of the vertical bit lines inthe y-direction and extending upwards through all the planes. Asignificant advantage of the structure of FIG. 7 is that all word linesand strips of insulation under them in a group of planes may be definedsimultaneously by use of a single mask, thus greatly simplifying themanufacturing process.

Referring to FIG. 7, a small part of four planes 101, 103, 105 and 107of the three-dimensional array are shown. Elements of the FIG. 7 arraythat correspond to those of the equivalent circuit of FIG. 1 areidentified by the same reference numbers. It will be noted that FIG. 7shows the two planes 1 and 2 of FIG. 1 plus two additional planes on topof them. All of the planes have the same horizontal pattern of gate,dielectric and memory storage element (NVM) material. In each plane,metal word lines (WL) are elongated in the x-direction and spaced apartin the y-direction. Each plane includes a layer of insulating dielectricthat isolates its word lines from the word lines of the plane below itor, in the case of plane 101, of the substrate circuit components belowit. Extending through each plane is a collection of metal local bit line(LBL) “pillars” elongated in the vertical z-direction and forming aregular array in the x-y direction.

Each bit line pillar is connected to one of a set of global bit lines(GBL) in the silicon substrate running in the y-direction at the samepitch as the pillar spacing through the select devices (Q_(xy)) formedin the substrate whose gates are driven by the select gate lines (SG)elongated in the x-direction, which are also formed in the substrate.The switching devices Q_(xy) may be conventional CMOS transistors (orvertical npn transistors) and fabricated using the same process as usedto form the other conventional circuitry. In the case of using npntransistors instead of MOS transistors, the select gate (SG) lines arereplaced with the base contact electrode lines elongated in thex-direction. Also fabricated in the substrate but not shown in FIG. 7are sense amplifiers, input-output (I/O) circuitry, control circuitry,and any other necessary peripheral circuitry. There is one select gateline (SG) for each row of local bit line pillars in the x-direction andone select device (Q) for each individual local bit line (LBL).

Each vertical strip of non-volatile memory element (NVM) material issandwiched between the vertical local bit lines (LBL) and a plurality ofword lines (WL) vertically stacked in all the planes. Preferably the NVMmaterial is present between the local bit lines (LBL) in thex-direction. A memory storage element (M) is located at eachintersection of a word line (WL) and a local bit line (LBL). In the caseof a metal oxide described above for the memory storage elementmaterial, a small region of the NVM material between an intersectinglocal bit line (LBL) and word line (WL) is controllably alternatedbetween conductive (set) and non-conductive (reset) states byappropriate voltages applied to the intersecting lines.

There may also be a parasitic NVM element formed between the LBL and thedielectric between planes. By choosing the thickness of the dielectricstrips to be large compared to the thickness of the NVM material layer(that is, the spacing between the local bit lines and the word lines), afield caused by differing voltages between word lines in the samevertical word line stack can be made small enough so that the parasiticelement never conducts a significant amount of current. Similarly, inother embodiments, the non-conducting NVM material may be left in placebetween adjacent local bit lines if the operating voltages between theadjacent LBLs remain below the programming threshold.

An outline of a process for fabricating the structure of FIG. 7 is asfollows:

-   -   1. The support circuitry, including the select devices Q, global        bit lines GBL, select gate lines SG and other circuits        peripheral to the array, is formed in the silicon substrate in a        conventional fashion and the top surface of this circuitry is        planarized, such as by etching with use of a layer of etch stop        material placed over the circuitry.    -   2. Alternating layers of dielectric (insulator) and metal are        formed as sheets on top of each other and over at least the area        of the substrate in which the select devices Q are formed. In        the example of FIG. 7, four such sheets are formed.    -   3. These sheets are then etched (isolated) by using a mask        formed over the top of them that has slits elongated in the        x-direction and spaced apart in the y-direction. All of the        material is removed down to the etch stop in order to form the        trenches shown in FIG. 7 in which the local bit line (LBL)        pillars and NVM material is later formed. Contact holes are also        etched through the etch stop material layer at the bottom of the        trenches to allow access to the drains of the select devices Q        at the positions of the subsequently formed pillars. The        formation of the trenches also defines the width in the        y-direction of the word lines (WL).    -   4. Non-volatile memory (NVM) material is deposited in thin        layers along the sidewalls of these trenches and across the        structure above the trenches. This leaves the NVM material along        the opposing sidewalls of each of the trenches and in contact        with the word line (WL) surfaces that are exposed into the        trenches.    -   5. Metal is then deposited in these trenches in order to make        contact with the non-volatile memory (NVM) material. The metal        is patterned using a mask with slits in the y-direction. Removal        of the metal material by etching through this mask leaves the        local bit line (LBL) pillars. The non-volatile memory (NVM)        material in the x-direction may also be removed between pillars.        The space between pillars in the x-direction is then filled with        a dielectric material and planarized back to the top of the        structure.

A significant advantage of the configuration of FIG. 7 is that only oneetching operation through a single mask is required to form the trenchesthrough all the layers of material of the planes at one time. However,process limitations may limit the number of planes that can be etchedtogether in this manner. If the total thickness of all the layers is toogreat, the trench may need to be formed in sequential steps. A firstnumber of layers are etched and, after a second number of layers havebeen formed on top of the first number of trenched layers, the toplayers are subjected to a second etching step to form trenches in themthat are aligned with the trenches in the bottom layers. This sequencemay be repeated even more times for an implementation having a verylarge number of layers.

A second example of implementing the three-dimensional memory cell arrayof FIG. 1 is illustrated by FIG. 8, and a process of forming thisstructure is outlined with respect to FIGS. 9-14. This structure isconfigured to use any type of material for the non-volatile memorystorage elements, electrically conductive or non-conductive whendeposited on the structure, such as those described above. The NVMelement is isolated from the LBL and is sandwiched between the bottommetal electrode and the word line. The bottom electrode makes electricalcontact with the LBL while the word line is electrically isolated fromthe LBL through an insulator. The NVM elements at the intersections ofthe local bit lines (LBL) and word lines (WL) are electrically isolatedfrom one another in the x and z-directions.

FIG. 8 shows a portion of each of three planes 111, 113 and 115 of thissecond structural example on only one side of a local bit line (LBL).The word lines (WL) and memory storage elements (M_(xy)) are defined ineach plane as the plane is formed, using two masking steps. The localbit lines crossing each plane of the group in the z-direction aredefined globally after the last plane in the group is defined. Asignificant feature of the structure of FIG. 8 is that the storageelements M_(xy) are below their respective word lines, rather thanserving as an insulator between the word lines (WL) and the verticallocal bit lines (LBL) as done in the example of FIG. 7. Further, abottom electrode contacts the lower surface of each storage elementM_(xy) and extends laterally in the y-direction to the local bit line(LBL). Conduction through one of the memory cells is through the bitline, laterally along the bottom electrode, vertically in thez-direction through the switching material of the storage elementsM_(xy) (and optional layer of barrier metal, if present) and to theselected word line (WL). This allows the use of conductive switchingmaterial for the storage elements M_(zxy) which in the example of FIG. 7would electrically short word lines in different planes which arevertically above each other. As shown in FIG. 8, the word lines (WL)stop short in the y-direction of the local bit lines (LBL) and do nothave the non-volatile memory (NVM) material sandwiched between the wordand local bit lines at the same z-location as is the case in the exampleof FIG. 7. The storage elements M_(xy) are similarly spaced from thelocal bit lines (LBL), being electrically connected thereto by thebottom electrode.

An outline of a process for forming one plane of the three-dimensionalstructure of FIG. 8 with storage elements M_(zxy) in a regular array inthe x-y direction is as follows:

-   -   a. Form, on a continuous dielectric (insulator) layer, parallel        sets of stacks containing strips of a bottom electrode,        switching material and (optionally) a barrier metal, wherein the        stacks are elongated in the y-direction and spaced apart in the        x-direction. This intermediate structure is shown in FIG. 9. The        process of forming this structure includes sequentially        depositing layers of the bottom insulator (to insulate the        device from the substrate in layer 111 and from lower planes in        layers 113 and 115), a bottom electrode of electrically        conducting material (for example, titanium), the switching NVM        material layer, a top electrode barrier metal (for example,        platinum), followed by a first layer of photoresist material.        Pattern the photoresist as a set of horizontal lines and spaces        running in the y-direction. The width of the photoresist lines        are reduced (the photoresist is “slimmed”) to reduce the width        of the lines of mask material so that the spaces between stacks        are larger than the width of the lines. This is to compensate        for a possible subsequent misalignment of the rows of switching        elements between different planes and to allow a common vertical        local bit line to make contact to the bottom electrode        simultaneously in all planes. This also reduces the size (and        thus current) of the switching elements. Using the photoresist        as a mask, the stack is etched, stopping on the bottom insulator        layer. The photoresist is then removed, and the gaps between        rows are filled with another insulator (not shown in FIG. 9) and        the resulting structure is planarized.    -   b. With reference to FIGS. 10-12, the stacks are separated to        form an x-y array of individual memory elements, each containing        a bottom electrode joining two adjacent memory elements in the        y-direction.        -   1. Deposit a layer of dielectric (insulator) over the            structure.        -   2. Pattern parallel lines of photoresist running in the            x-direction and etch the top insulator layer to form from            this layer the parallel strips of insulation I1 shown in            FIG. 10. This etching is stopped on the barrier metal (or            memory material if the barrier metal is not present) and the            insulator filling the gaps between the stacks (not shown).    -   3. Exposed areas of the array thus formed are filled with a        second insulator (12) with different etching properties than        insulator I1, which is then planarized. The result is        illustrated in FIG. 11.    -   4. Thereafter, all remaining insulator I1 is removed by        selective etching that uses the exposed I2 as a mask. Spacers        are then formed along the edges of I2 as illustrated in FIG. 12.    -   5. Using the spacers and the I2 strips as a mask, the parallel        stacks are etched through, including the bottom electrode        strips, thereby isolating the bottom electrode strips by        trenches between them so that each strip contacts only two        adjacent memory elements M_(zxy). As an alternative to forming        the spacers for use as part of the etch mask, a photoresist mask        may be formed instead. However, there is a potential of        misalignment of such a photoresist mask and its pitch may not be        as small as can be obtained with the user of the spacers.    -   6. A third insulator layer is then deposited over the structure        and into the trenches just etched, and the third insulator layer        is etched back to slightly above the height of the exposed        switching material, thereby leaving the third insulators I3. The        result is shown in FIG. 12, a cross-section drawn in the        y-direction along one bottom electrode line.    -   c. The word lines are then formed in the exposed region, making        ohmic contact to two adjacent memory elements (this is a        Damascene process).        -   1. The spacers are first removed. The result is shown as            FIG. 13, a rectangular x-y array of memory stacks (like            upward facing pillars), each two adjacent stacks in the            y-direction being connected by a common bottom electrode.            Not shown for clarity is the insulator I2 filling the area            over the bottom electrode between pillars, and the insulator            I3 filling the trench between the gaps separating the bottom            electrodes and adjacent pillars.        -   2. Conductive word line material is then deposited, and is            removed by CMP so that it fills the exposed trench, stopping            on insulator I3 and barrier metal (if present) or memory            material. Note that the insulator I2 forms a trench where            the conductive word line material is defined (as a damascene            process). The word lines (WL) sit over insulator I3 and two            adjacent memory stacks (shown here with barrier metal). The            resulting structure is shown in FIG. 14.    -   d. The foregoing processing steps are repeated for each plane in        the group of planes. Note that the memory elements in one plane        will not be exactly aligned with memory elements in another        plane because of photolithography misalignment.    -   e. After the circuit elements of all the planes have been        formed, the vertical local bit lines are then formed:        -   1. A top insulator is deposited above the word lines of the            upper plane.        -   2. Using a photoresist mask, an x-y “contact” pattern is            opened for the individual local bit lines, and etching is            performed through the group of planes all the way to the            substrate. Rows of these openings are aligned parallel to            the word lines along the x-direction but are spaced midway            in the gaps between word lines in the y-direction. The size            of these openings is smaller than the spacing between word            lines and aligned in the x-direction to cut through the            bottom electrodes in each plane. As the etch moves through            each layer of bottom electrodes of the several planes, it            separates the bottom electrodes into two segments so that            each segment contacts only one memory element. The etching            continues to the substrate where it exposes contacts to the            select devices Q_(xy).        -   3. These holes are then filled with metal to form the local            bit lines, and the top surface is planarized so that each            local bit line is independent of (electrically separated            from) any other local bit line. A barrier metal may be            optionally deposited as a part of this process. The            resulting structure is shown in the vertical cross-section            of FIG. 8.        -   4. Alternatively, instead of etching an x-y “contact”            pattern for the local bit lines, slits elongated in            x-direction and spaced apart in the y-direction are etched            in the I2 oxide regions. Etching is performed through the            group of planes, all the way to the substrate forming            trenches in which the local bit line pillars are later            formed.        -   5. Metal is then deposited to fill these trenches. The            deposited metal makes contact with the bottom electrode of            the memory element in all the planes. The metal is then            patterned using a mask with slits in the x-direction.            Removal of the metal material by etching through this mask            leaves the local bit line pillars. The space between pillars            in the x-direction is filled with a dielectric material and            planarized back to the top of the structure.

A third specific structural example is shown by FIG. 15, which shows asmall portion of three planes 121, 123 and 125. The memory storageelements M_(zxy) are also formed from a conductive switching material.This is a variation of the second example, wherein the memory elementsof FIG. 15 individually takes the shape of the bottom electrode andcontacts the vertical local bit line (LBL). The bottom electrodes of theexample of FIG. 8 are missing from the layers shown in FIG. 15.

The structure shown in FIG. 15 is made by essentially the same processas that described above for the second example. The main difference isthat in the second example, reference to the bottom electrode isreplaced in this third example by the switching material, and referenceto the switching material of the second embodiment is not used in thisthird embodiment.

The second example structure of FIG. 8 is particularly suited to anyswitching material that as deposited as an insulator or electricalconductor. The third example structure shown in FIG. 15 is suitedprimarily for switching materials that are deposited as an electricalconductor (phase change materials, carbon materials, carbon nanotubesand like materials). By isolating the switching material such that itdoes not span the region between two stacks, the possibility of aconductive short between switching elements is eliminated.

Embodiments with Reduced Leakage Currents

Conventionally, diodes are commonly connected in series with thevariable resistive elements of a memory array in order to reduce leakagecurrents that can flow through them. The highly compact 3Dreprogrammable memory described in the present invention has anarchitecture that does not require a diode in series with each memoryelement while able to keep the leakage currents reduced. This ispossible with short local vertical bit lines which are selectivelycoupled to a set of global bit lines. In this manner, the structures ofthe 3D memory are necessarily segmented and couplings between theindividual paths in the mesh are reduced.

Even if the 3D reprogrammable memory has an architecture that allowsreduced current leakage, it is desirable to further reduce them. Asdescribed earlier and in connection with FIG. 5, parasitic currents mayexist during a read operation and these currents have two undesirableeffects. First, they result in higher power consumption. Secondly, andmore seriously, they may occur in the sensing path of the memory elementbeing sensed, cause erroneous reading of the sensed current.

FIG. 16 illustrates the read bias voltages and current leakage acrossmultiple planes of the 3D memory shown in FIG. 1 and FIG. 3. FIG. 16 isa cross-sectional view across 4 planes along the x-direction of aportion of the perspective 3D view of the memory shown in FIG. 1. Itshould be clear that while FIG. 1 shows the substrate and 2 planes, FIG.16 shows the substrate and 4 planes to better illustrate the effect ofcurrent leakage from one plane to another.

In accordance with the general principle described in connection withFIG. 5, when the resistive state of a memory element 200 in FIG. 16 isto be determined, a bias voltage is applied across the memory elementand its element current I_(ELEMENT) sensed. The memory element 200resides on Plane 4 and is accessible by selecting the word line 210(Sel-WLi) and the local bit line 220 (Sel-LBLj). For example, to applythe bias voltage, the selected word line 210 (Sel-WLi) is set to 0v andthe corresponding selected local bit line 220 (Sel-LBLj) is set to areference such as 0.5V via a turned on select gate 222 by a senseamplifier 240. With all other unselected word line in all planes alsoset to the reference 0.5V and all unselected local bit lines also set tothe reference 0.5V, then the current sensed by the sense amplifier 240will just be the I_(ELEMENT) of the memory element 200.

The architecture shown in FIG. 1 and FIG. 16 has the unselected localbit lines (LBLj+1, LBLj+2, . . . ) and the selected local bit line(Sel-LBLj) all sharing the same global bit line 250 (GBLi) to the senseamplifier 240. During sensing of the memory element 200, the unselectedlocal bit lines can only be isolated from the sense amplifier 240 byhaving their respective select gate such as gate 232 turned off. In thisway, the unselected local bit lines are left floating and will couple tothe reference 0.5V by virtue of adjacent nodes which are at 0.5V.However, the adjacent nodes are not exactly at the reference 0.5V. Thisis due to a finite resistance in each word line (perpendicular to theplane in FIG. 16) which results in a progressive voltage drop away fromone end of the word line at which 0.5V is applied. This ultimatelyresults in the floating, adjacent unselected local bit lines coupling toa voltage slightly different from the reference 0.5V. In this instance,there will be leakage currents between the selected and unselected localbit lines as illustrated by broken flow lines in FIG. 16. Then sensedcurrent is then I_(ELEMENT)+leakage currents instead of justI_(ELEMENT). This problem becomes worse will increasing word line'slength and resistivity.

Double-Global-Bit-Line Architecture

According to one aspect of the invention, a 3D memory includes memoryelements arranged in a three-dimensional pattern defined by rectangularcoordinates having x, y and z-directions and with a plurality ofparallel planes stacked in the z-direction. The memory elements in eachplane are accessed by a plurality of word lines and local bit lines intandem with a plurality of global bit lines. The plurality of local bitlines are in the z-direction through the plurality of planes andarranged in a two dimensional rectangular array of rows in thex-direction and columns in the y-directions. The plurality of word linesin each plane are elongated in the x-direction and spaced apart in they-direction between and separated from the plurality of local bit linesin the individual planes. A non-volatile, reprogramming memory elementis located near a crossing between a word line and local bit line andaccessible by the word line and bit line and wherein a group of memoryelements are accessible in parallel by a common word line and a row oflocal bit lines. The 3D memory further includes a double-global-bit linearchitecture with two global bit lines respectively serving even and oddlocal bit lines in a column thereof in the y-direction. Thisarchitecture allows one global bit line to be used by a sense amplifierto access a selected local bit line and the other global bit line to beused to access an unselected local bit lines adjacent the selected localbit line in the y-direction. In this way the adjacent, unselected locallines can be set to exactly a reference voltage same as that of theselected local bit line in order to eliminate leakage currents betweenadjacent bit lines.

FIG. 17 illustrates a three-dimensional memory with adouble-global-bit-line architecture for improved access to a set oflocal bit lines. An architecture of a three-dimensional memory 10′ isschematically and generally illustrated in the form of an equivalentcircuit of a portion of such a memory. This is a specific example of thethree-dimensional array summarized above. A standard three-dimensionalrectangular coordinate system 11 is used for reference, the directionsof each of vectors x, y and z being preferably orthogonal with the othertwo and having a plurality of parallel planes stacked in thez-direction. The local bit lines are elongated vertically, in thez-direction, and form a regular two-dimensional array in the x (row) andy (column) directions.

Memory storage elements M_(zxy) are formed in a plurality of planespositioned at different distances in the z-direction above the substrate13. Two planes 1 and 2 are illustrated in FIG. 17 but there willtypically be more, such as 4, 6 or even more. In each plane at distancez, word lines WL_(zy) are elongated in the x-direction and spaced apartin the y-direction between the local bit-lines (LBL_(xy)). Each row oflocal bit lines LBL_(xy) of each plane is sandwiched by a pair of wordlines WL_(zy) and WL_(zy+1). Individually crossing between a local bitline a word line occurs at each plane where the local bit lineintersects the plane. The individual memory storage elements M_(zxy) areconnected between one local bit line LBL_(xy) and one word line WL_(zy)adjacent these individual crossings. An individual memory elementM_(zxy) is therefore addressable by placing proper voltages on the localbit line LBL_(xy) and word line WL_(zy) between which the memory elementis connected. The voltages are chosen to provide the electrical stimulusnecessary to cause the state of the memory element to change from anexisting state to the desired new state. The levels, duration and othercharacteristics of these voltages depend upon the material that is usedfor the memory elements.

Each “plane” of the three-dimensional memory cell structure is typicallyformed of at least two layers, one in which the conductive word linesWL_(zy) are positioned and another of a dielectric material thatelectrically isolates the planes from each other. Additional layers mayalso be present in each plane, depending for example on the structure ofthe memory elements M_(zxy). The planes are stacked on top of each otheron a semiconductor substrate with the local bit lines LBL_(xy) beingconnected with storage elements M_(zxy) of each plane through which thelocal bit lines extend.

Essentially the three-dimensional memory 10′ shown in FIG. 17 is similarto the 3D memory 10 shown in FIG. 1 except for the structure of theglobal bit lines which has a doubling of the global bit lines.

A circuit for selectively connecting internal memory elements withexternal data circuits is preferably formed in a semiconductor substrate13. In this specific example, a two-dimensional array of select orswitching devices Q_(xy) are utilized, where x gives a relative positionof the device in the x-direction and y its relative position in they-direction. The individual devices Q_(xy) may be a select gate orselect transistor, as examples.

A pair of global bit lines (GBL_(xA), GBL_(xB)) is elongated in they-direction and have relative positions in the x-direction that areindicated by the subscript. The individual devices Qxy each couples alocal bit line to one global bit line. Essentially, each local bit linein a row is coupleable to one of a corresponding pair of global bitlines. Along a column of local bit lines, even local bit lines arecoupleable to a first one of a corresponding pair of global bit linewhile odd local bit lines are coupleable to a second one of thecorresponding pair of global bit line.

Thus, a pair of global bit lines (GBL_(x′A), GBL_(x′B)) at about thex′-position, are individually connectable with the source or drain ofthe select devices Q in such a manner that local bits (LBLx′y) at thex′-position and along the y-direction are coupleable alternately to thepair of global bit lines (GBL_(x′A), GBL_(x′B)). For example, the oddlocal bit lines along the column in the y-direction at the x=1 position(LBL₁, LBL₁₃, . . . ) are coupleable respectively via select devices(Q₁₁, Q₁₃, . . . ) to a first one GBL_(1A) of the pair of global bitline at x=1. Similarly, the even local bit lines along the same columnat the x=1 position (LBL₁₂, LBL₁₄, . . . ) are coupleable respectivelyvia select devices (Q₁₂, Q₁₄, . . . ) to a second one GBL_(1B) of thepair of global bit line at x=1.

During reading and also typically programming, each global bit line istypically coupled to one local bit line by accessing through acorresponding select device that has been turned on. In this way a senseamplifier can access the local bit line via the coupled global bit line.

In order to connect one set (in this example, designated as one row) oflocal bit lines with a corresponding set of global bit lines, controlgate lines SG_(y) are elongated in the x-direction and connect withcontrol terminals (gates) of a single row of select devices Q_(xy)having a common position in the y-direction. In this way, a set or pageof memory elements can be accessed in parallel. The select devicesQ_(xy) therefore connect one row of local bit lines (LBL_(xy)) acrossthe x-direction (having the same position in the y-direction) at a timeto corresponding ones of the global bit-lines, depending upon which ofthe control gate lines SG_(y) receives a voltage that turns on theselect devices to which it is connected. In the double-global-bit linearchitecture, there is a pair of global bit lines at about eachx-position. If a row of local bit lines along the x-directions arecoupleable to the first one of each pair of corresponding global bitlines, then along the y-direction, an adjacent row of local bit lineswill be coupleable to the second one of each pair of correspondingglobal bit lines. For example, the row of local bit lines (LBL₁₁, LBL₂₁,LBL₃₁, . . . ) along the x-direction are coupled to the first of eachpair of corresponding global bit lines (GBL_(1A), GBL_(2A), GBL_(3A), .. . ) by turning on select devices (Q₁₁, Q₂₁, Q₃₁, . . . ) via thecontrol gate line SG₁. Along the y-direction, an adjacent row of localbit lines (LBL₁₂, LBL₂₂, LBL₃₂, . . . ) along the x-direction arecoupled to the second of each pair of corresponding global bit lines(GBL_(1B), GBL_(2B), GBL_(3B), . . . ) by turning on select devices(Q₁₂, Q₂₂, Q₃₂, . . . ) via the control gate line SG₂. Similarly, a nextadjacent row of local bit lines (LBL₁₃, LBL₂₃, LBL₃₃, . . . ) arecoupled to the first of each pair of corresponding global bit lines(GBL_(1A), GBL_(2A), GBL_(3A), . . . ) in an alternating manner betweenthe first and second one of each pair.

By accessing a row of local bit lines and an adjacent row usingdifferent ones of each pair of corresponding global bit lines, the rowand adjacent row of local bit lines can be accessed independently at thesame time. This is in contrast to the case of the single-global-bit-linearchitecture shown in FIG. 1, where both a row and its adjacent row oflocal bit lines share the same corresponding global bit lines.

As discussed in connection with FIG. 16, the leakage currents due toadjacent rows are not well controlled when the adjacent bit lines cannotbe set independently to the reference voltage in order to eliminatecurrent leakage.

FIG. 18 illustrates the elimination of leakage currents in thedouble-global-line architecture 3D array of FIG. 17. The analysis ofleakage current is similar to that described with respect to FIG. 16.However, with the double-global-bit-line architecture, the selectedlocal bit line 220 (Sel-LBLj) allows the memory element 200 to be sensedby the sense amplifier 240 via the first one of the pair of global bitline GBL_(iA), which is maintained at a reference voltage (e.g., 0.5V).At the same time, the adjacent local bit line 230 can be accessedindependently by the second one of the pair of global bit line GBL_(iB).This allows the adjacent local bit line 230 to be set to the samereference voltage. Since both the selected local bit line 220 and itsadjacent local bit line (along the y-direction) are at the samereference voltage, there will be no leakage currents between the twolocal bit lines adjacent to each other.

The double-global-bit-line architecture doubles the number of global bitlines in the memory array compared to the architecture shown in FIG. 1.However, this disadvantage is offset by providing a memory array withless leakage currents among the memory elements.

Single-Sided Word Line Architecture

According to another embodiment of the invention, a 3D memory includesmemory elements arranged in a three-dimensional pattern defined byrectangular coordinates having x, y and z-directions and with aplurality of parallel planes stacked in the z-direction. The memoryelements in each plane are accessed by a plurality of word lines andlocal bit lines in tandem with a plurality of global bit lines. Theplurality of local bit lines are in the z-direction through theplurality of planes and arranged in a two dimensional rectangular arrayof rows in the x-direction and columns in the y-directions. Theplurality of word lines in each plane are elongated in the x-directionand spaced apart in the y-direction between and separated from theplurality of local bit lines in the individual planes. A non-volatile,reprogramming memory element is located near a crossing between a wordline and local bit line and accessible by the word line and bit line andwherein a group of memory elements are accessible in parallel by acommon word line and a row of local bit lines. The 3D memory has asingle-sided word line architecture with each word line exclusivelyconnected to one row of memory elements. This is accomplished byproviding one word line for each row of memory elements instead ofsharing one word line between two rows of memory elements and linkingthe memory element across the array across the word lines. While the rowof memory elements is also being accessed by a corresponding row oflocal bit lines, there is no extension of coupling for the row of localbit lines beyond the word line.

A double-sided word line architecture has been described earlier in thateach word line is connected to two adjacent rows of memory elementsassociated with two corresponding rows of local bit lines, one adjacentrow along one side of the word line and another adjacent row along theother side. For example, as shown in FIG. 1 and FIG. 3, the word lineWL₁₂ is connected on one side to a first row (or page) of memoryelements (M₁₁₄, M₁₂₄, M₁₃₄, . . . ) associated respectively with localbit lines (LBL₁₂, LBL₂₂, LBL₃₂, . . . ) and also connected on anotherside to a second row (or page) of memory elements (M₁₁₅, M₁₂₅, M₁₃₅, . .. ) associated respectively with local bit lines (LBL₁₃, LBL₂₃, LBL₃₃, .. . )

FIG. 19 illustrates schematically a single-sided word line architecture.Each word line is connected to an adjacent row of memory elementsassociate with one row of local bit lines on only one side.

The 3D memory array with the double-sided word line architectureillustrated in FIG. 1 can be modified to the single-sided word linearchitecture where each word line except ones at an edge of the arraywill be replaced by a pair of word lines. In this way, each word line isconnecting exclusively to one row of memory elements. Thus, the wordline WL₁₂ shown in FIG. 1 is now replaced in FIG. 19 by the pair of wordlines WL₁₃ and WL₁₄. It will be seen that WL13 is connected to one rowof memory elements (M₁₁₄, M₁₂₄, M₁₃₄, . . . ) and WL14 is connected toone row of memory elements (M₁₁₅, M₁₂₅, M₁₃₅, . . . ) As describedbefore, a row of memory elements constitutes a page which is read orwritten to in parallel.

FIG. 20 illustrates one plane and substrate of the 3D array with thesingle-sided word line architecture. Going from the double-sided wordline architecture of FIG. 3, similarly, WL₁₂ in FIG. 3 would be replacedby the pair WL₁₃, WL₁₄ in FIG. 20, etc. In FIG. 3, a typicaldouble-sided word line (e.g., WL₁₂) is connected to two rows of memoryelements (on both side of the word line). In FIG. 20, each single-sidedword line (e.g., WL₁₃) is connected to only one row of memory elements.

FIG. 20 also illustrates a minimum block of memory elements that iserasable as a unit to be defined by two row of memory elements (M₁₁₃,M₁₂₃, M₁₃₃, . . . ) and (M₁₁₄, M₁₂₄, M₁₃₄, . . . ) sharing the same rowof local bit lines (e.g., LBL₁₂, LBL₂₂, LBL₃₂, . . . )

FIG. 21 illustrates the elimination of leakage currents in thesingle-sided word-line architecture 3-D array of FIGS. 19 and 20. Theanalysis of leakage current is similar to that described with respect toFIG. 16. However, with the single-sided word-line architecture, theselected local bit line 220 (Sel-LBLj) is not coupled to an adjacent bitline 230 across the separated word lines 210 and 212. Thus there is noleakage current between adjacent local bit lines and the sense currentin the sense amplifier 240 via the global bit line 250 and the local bitline 220 will be just that from the current of the memory elementI_(ELEMENT).

The single-sided word-line architecture doubles the number of word linesin the memory array compared to the architecture shown in FIG. 1.However, this disadvantage is offset by providing a memory array withless leakage currents among the memory elements.

FIG. 22 is an isometric view of a portion of the 3D array with thesingle-sided word line architecture shown in FIG. 19. Again, similar tothe isometric view for the double-side word line architecture shown inFIG. 7, FIG. 22 is one specific example of implementation for thesingle-sided word-line architecture. The main difference compared toFIG. 7 is that each word line is connected to one side to a row ofmemory elements. As explained earlier, this architecture has theadvantage of decoupling bit-line to bit line coupling across theplurality of word lines in the y-direction.

The 3D array is configured for use of memory element (NVM) material thatis non-conductive when first deposited. A metal oxide of the typediscussed earlier has this characteristic. As explained with respect toFIG. 6, conductive filaments are formed between electrodes on oppositesides of the material in response to appropriate voltages placed onthose electrodes. These electrodes are a bit line and a word line in thearray. Since the material is otherwise non-conductive, there is nonecessity to isolate the memory elements at the cross-points of the wordand bit lines from each other. Several memory elements may beimplemented by a single continuous layer of material, which in the caseof FIG. 22 are strips of NVM material oriented vertically along oppositesides of the vertical bit lines in the y-direction and extending upwardsthrough all the planes. A significant advantage of the structure of FIG.22 is that all word lines and strips of insulation under them in a groupof planes may be defined simultaneously by use of a single mask, thusgreatly simplifying the manufacturing process.

Referring to FIG. 22, a small part of four planes 101, 103, 105 and 107of the three-dimensional array are shown. Elements of the FIG. 22 arraythat correspond to those of the equivalent circuit of FIG. 19 areidentified by the same reference numbers. It will be noted that FIG. 22shows the two planes 1 and 2 of FIG. 19 plus two additional planes ontop of them. All of the planes have the same horizontal pattern of wordlines, dielectric and memory storage element (NVM) material. In eachplane, metal word lines (WL) are elongated in the x-direction and spacedapart in the y-direction. Each plane includes a layer of insulatingdielectric that isolates its word lines from the word lines of the planebelow it or, in the case of plane 101, of the substrate circuitcomponents below it. Extending through each plane is a collection ofmetal local bit line (LBL) “pillars” elongated in the verticalz-direction and forming a regular array in the x-y direction.

Each bit line pillar is connected to one of a set of global bit lines(GBL) in the silicon substrate running in the y-direction at the samepitch as the pillar spacing through the select devices (Q_(xy)) formedin the substrate whose gates are driven by the select gate lines (SG)elongated in the x-direction, which are also formed in the substrate.The switching devices Q_(xy) may be conventional CMOS transistors (orvertical npn transistors) and fabricated using the same process as usedto form the other conventional circuitry. In the case of using npntransistors instead of MOS transistors, the select gate (SG) lines arereplaced with the base contact electrode lines elongated in thex-direction. Also fabricated in the substrate but not shown in FIG. 22are sense amplifiers, input-output (I/O) circuitry, control circuitry,and any other necessary peripheral circuitry. There is one select gateline (SG) for each row of local bit line pillars in the x-direction andone select device (Q) for each individual local bit line (LBL).

Each vertical strip of non-volatile memory element (NVM) material issandwiched between the vertical local bit lines (LBL) and a plurality ofword lines (WL) vertically stacked in all the planes. Preferably the NVMmaterial is present between the local bit lines (LBL) in thex-direction. A memory storage element (M) is located at eachintersection of a word line (WL) and a local bit line (LBL). In the caseof a metal oxide described above for the memory storage elementmaterial, a small region of the NVM material between an intersectinglocal bit line (LBL) and word line (WL) is controllably alternatedbetween conductive (set) and non-conductive (reset) states byappropriate voltages applied to the intersecting lines.

There may also be a parasitic NVM element formed between the LBL and thedielectric between planes. By choosing the thickness of the dielectricstrips to be large compared to the thickness of the NVM material layer(that is, the spacing between the local bit lines and the word lines), afield caused by differing voltages between word lines in the samevertical word line stack can be made small enough so that the parasiticelement never conducts a significant amount of current. Similarly, inother embodiments, the non-conducting NVM material may be left in placebetween adjacent local bit lines if the operating voltages between theadjacent LBLs remain below the programming threshold.

The single-sided word line architecture almost double the number of wordline in the memory array compared to the double-sided one. Thisdisadvantage is offset by providing a more partitioned memory array withless leakage currents among the memory elements.

While the exemplary embodiments have been described using a 3Dco-ordinate system preferably with orthogonal axes, other embodiment inwhich the local bit lines LBL, word lines WL and global bit lines GBLcross at angles different than 90 degrees are also possible andcontemplated.

CONCLUSION

Although the various aspects of the present three-dimensionalnon-volatile memory and methods have been described with respect toexemplary embodiments thereof, it will be understood that the presentmemory and methods are entitled to protection within the full scope ofthe appended claims.

It is claimed:
 1. A data memory, comprising: a plurality of non-volatilere-programmable memory elements arranged in a three dimensional patterndefined by rectangular coordinates along x, y and z directions and witha plurality of parallel planes stacked in the z direction on top of asemiconductor substrate; a plurality of first conductive lines elongatedin the z direction through the plurality of planes and arranged in a twodimensional rectangular array of rows in the x direction and columns inthe y-direction; a plurality of second conductive lines elongated in thex-direction across each of the planes and spaced apart in they-directionbetween and separated from the first plurality of conductive lines ineach of the planes, wherein the first and second conductive lines crossadjacent each other at a plurality of locations across each of theplanes, the plurality of second conductive lines forming pairs ofadjacent second conductive lines, wherein each non-volatilere-programmable memory element is connected between a correspondingfirst conductive line and a corresponding second conductive linecrossing each other at an associated location of the plurality oflocations; and a plurality of select devices arranged to individuallycouple a selected row of first conductive lines to respective sensingcircuits via a plurality of third conductive lines, wherein, for each ofthe pairs of adjacent second conductive lines, each second conductiveline is connected to only one corresponding row of first conductive linesuch that both of the second conductive lines of each pair are connectedto the same row of first conductive lines and such that each connectionmade with each first conductive line of the corresponding row of firstconductive lines is via a single corresponding non-volatilere-programmable memory element, and wherein each memory element ispositioned between a respective first conductive line and a respectivesecond conductive line, and is in the plane of said plurality of planeswhere said respective second conductive line is placed, to contact thefirst conductive line in the y-direction.
 2. The data memory of claim 1,wherein: the third conductive lines are elongated in the y-direction,the select devices are arranged to make those of the first conductivelines aligned in the y-direction connectable with selected ones of theplurality of third conductive lines, and a plurality of control linesextending in the x-direction and individually connected with a pluralityof the select devices aligned in the x-direction to enable connection ofa plurality of first conductive lines aligned in the x-direction withdifferent ones of the third conductive lines.
 3. The data memory ofclaim 2, wherein the plurality of select devices and the plurality ofthird conductive lines are formed in the semiconductor substrate and theplurality of planes are formed as a stack over the semiconductorsubstrate.
 4. The data memory of claim 1, wherein the plurality ofselect devices and the plurality of third conductive lines are formed inthe semiconductor substrate and the plurality of planes are formed as astack over the semiconductor substrate.
 5. The data memory of claim 4,wherein the individual memory elements are characterized by including amaterial that reversibly changes its level of electrical conductancebetween at least first and second stable levels in response to anelectrical stimulus being applied through the first and secondconductive lines between which the memory element is connected.
 6. Thedata memory of claim 1, wherein the individual memory elements arecharacterized by including a material that reversibly changes its levelof electrical conductance between at least first and second stablelevels in response to an electrical stimulus being applied through thefirst and second conductive lines between which the memory element isconnected.
 7. The data memory of claim 1, wherein memory elementsinclude individual quantities of material adjacent individual ones ofthe crossings of the first and second conductive lines, the memoryelements being separated from one another in all of the x, y andz-directions.
 8. The data memory of claim 1, wherein the memory elementsare additionally positioned to contact the first conductive lines in they-direction.
 9. The data memory of claim 1, wherein the individualmemory elements are characterized by a level of electrical conductancethat changes in response to an electrical stimulus applied theretothrough the first and second conductive lines.
 10. The data memory ofclaim 1, additionally comprising a circuit connected to the first andsecond conductive lines to apply electrical stimuli to selected ones ofthe memory elements, the electrical stimuli being applied to cause theselected memory elements to switch from their first to second stablestates having substantially the same magnitude but different polaritiesas the electrical stimuli applied to cause the memory elements to switchfrom their second to first stable states.
 11. The data memory of claim1, additionally comprising data input-output circuits connected to thethird plurality of conductive lines.
 12. The data memory of claim 11,wherein the data input-output circuits include a plurality of senseamplifiers that are connected with the third conductive lines in amanner to provide a binary representation of data carried by the thirdconductive lines when data are being read from the memory.
 13. The datamemory of claim 11, wherein the data input-output circuits additionallyinclude data programming circuits that apply voltages to selected onesof the third conductive lines for programming data into at least some ofthe memory elements connected thereto through the plurality of selectdevices.
 14. A method of operating a re-programmable non-volatile memorysystem, comprising: utilizing at least one integrated circuit thatincludes a three-dimensional pattern of memory elements defined byrectangular coordinates along x, y and z-directions and which includes:a plurality of parallel planes stacked in the z-direction on top of asemiconductor substrate, a plurality of conductive local bit lineselongated in the z-direction through the plurality of planes andarranged in a two-dimensional rectangular array of rows in the x andcolumns in they-directions, a plurality of word lines elongated in thex-direction across each of the planes and spaced apart in they-directionbetween and separated from the plurality of local bit lines in each ofthe planes, wherein the local bit lines and word lines cross adjacenteach other at a plurality of locations across each of the planes, theplurality of word lines forming pairs of adjacent word lines, aplurality of re-programmable non-volatile memory elements, eachnon-volatile re-programmable memory element being connected between acorresponding local bit line and a corresponding word line crossing eachother at an associated location of the plurality of locations, and aplurality of select devices arranged to individually couple a selectedrow of local bit lines to respective sensing circuits via a plurality ofglobal bit lines in response to a select control signal; applying saidselect control signal to the plurality of select devices in order toconnect the selected row of local bit lines to individual ones of theglobal bit lines; and causing a selected one or more of the plurality ofmemory elements to simultaneously change between at least first andsecond states by applying one of first and second stimuli through one ormore of the plurality of word lines and global bit lines between whichthe selected one or more of the plurality of memory elements areoperably connected, wherein, for each of the pairs of adjacent wordlines, each word line is connected to only one corresponding row oflocal bit lines, such that both of the word lines of each pair areconnected to the same row of local bit lines and such that eachconnection made with each local bit line of the corresponding row oflocal bit lines is via a single corresponding non-volatilere-programmable memory element, and wherein each memory element ispositioned between a respective local bit line and a respective wordline to contact the respective local bit line in the y-direction. 15.The method of claim 14, wherein causing a selected one or more of theplurality of memory elements to simultaneously change between their atleast first and second states includes simultaneously resetting to thefirst state two rows of memory elements connected to the selected row oflocal bit lines and along opposite sides thereof in the y-direction byapplying the second electrical stimulus to a plurality of the global bitlines and to two of the word lines adjacent the selected row of localbit lines on said opposite sides thereof.
 16. The method of claim 15,wherein causing a selected one or more of the plurality of memoryelements to simultaneously change between their at least first andsecond states additionally includes subsequently programming data intoone of the two rows of reset memory elements by applying the firstelectrical stimulus to a plurality of the global bit lines and one ofthe word lines adjacent the selected row of local bit lines on the sidethereof of the one row of memory elements being programmed.
 17. Themethod of claim 15, additionally comprising reading the states of a rowof memory elements along one side in the y-direction of the selected rowof local bit lines by applying reading electrical stimuli to the globalbit lines and a word line such that the states of the memory elementsare read from an electrical quantity appearing on the global bit lines.18. The method of claim 15, wherein causing a selected one or more ofthe plurality of memory elements to simultaneously change between theirat least first and second states additionally includes applying aplurality of pulses of the one of the first and second stimuli and, inbetween successive pulses, verifying the state of the one or more of theplurality of memory elements.